In order to be famous, Gaga, Katy and Madonna had a tough road.
In the music industry itself, Gaga, Madonna, Mariah, Britney Beyoncé, Taylor and Katy work very hard. They are constantly reinventing themselves and their stages, songwriting skills and productions are just amazing

Ultraviolence - 75/100, not bad but not one of my faves at all
The music video for Shades Of Cool remind me of End Game It doesn't fit the nice song, which deserves a 78/100 from me
Fucking Desire 76/100
Ugh her music is angelic yet depressing, not something I would listen everyday

Video Games ia a gem, now I'll continue with her newest single, Hope Is A Dangerous Thing For A Woman Like Me To Have - But I Have It (Look What You Made Me Do, We Are Never Ever Getting Back Together and This Is Why We Can't Have Nice Things are shaking)

Yes, Shake It Off did this without Spotify tho. Having strong pure sales for months can give you longevity as well but I think Ariana needs the streams more..
It's like she gets hits that chart for around 25 weeks but we need that one charting over at least 40 weeks

She is likely to win at least one Grammy this year
She won Artist of The Year twice at the American Music Awards (2015 and 2016, possibly 2019 as well)
She was the Woman of The Year by Billboard in 2018
She is nominated for International Female Solo Artist this year
She has won two MTV Video Music Awards, one for Best Pop Video (Problem) and one for Best Pop (No Tears Left To Cry)
She has 11 top 10 hits including 6 top 5 hits, 4 top 3 hits and 1 #1 by far
She has sold around 40 million copies in the US
Her net worth on the other hand
I get your point, she is a late bloomer which makes people believe she's already a legend. Making decent hit after decent (top 5 or not) doesn't make you a legend indeed, but Thank U, Next could become her first iconic hit if it remains at least 35 weeks on the charts. She is a very global artists and she makes earworms, which makes her a real single-artist (this is particularly the reasons why her albums don't sell that great). She may be the next main pop girl but she has no competition so being a bit glaring will give you easily a #1 considering the weak sales these days and a current lack of pioneers like Taylor, Katy, Gaga (Shallow is from October), Beyoncé, Madonna and Rihanna.
Ariana needs a global smash like, Roar, Shake It Off, Uptown Funk or Shape Of You. Thank U, Next seems the closest option, but the music industry is so fast these days and I wouldn't be too surprised if it stays on the charts for only 25 weeks (this can be seen as long, but the songs I've mentioned are diamond). It should be at least 35 weeks on several charts like the ones I named did. Otherwise she's just that girl who was lucky with a #1 and a lot of decent hits. However, I don't know if she wants to be seen as a legend. Her stuff is selling but lyrically and sonically it is starting to sound very generic and lackless (doesn't mean I don't bop to her current ones), basically what nowadays rappers do.
On the other hand, I have high expectations for this era since 7 Rings got almost 15 million streams within one day which means it's likely to go #1 this month, making it her 2nd #1 in a short time and her 2nd #1 in overall. She won't fade away soon I also feel like the album Thank U, Next will sell better than her previous albums since she seems to have more power at the moment. My predictions are that the album will sell around 700k sps in the first week (Sweetener sold around 400k) and 400k in the US (Sweetener sold around 200k) and around 4 million within one year (comparible with A Star Is Born)
Also, Ariana needs better touring sales to be considered iconic. We'll see what the Sweetener World Tour brings, I hope it will gross at least 100 million dollars Mariah had 18 #1s and a great voice, Ariana has 1 #1 by far and a great voice. She isn't something reinventing. However, she could experiment more with different genres like she is already doing now to attract people's attention even more and get a couple more #1s.
At the end of the day, Ariana's working hard and Miley will probably release her next album this year. We have to see what Selena does